The field of the invention generally pertains to dough sheeters and methods of sheeting dough for making flattened food products, such as tortillas, tortilla chips, etc. The invention relates more particularly to a dough pre-sheeting system and method which extrudes relatively thin sheets of dough in preparation for further sheeting and processing within a flattened food product processing system.
Tortillas and tortilla products, such as tortilla chips, are flat food products made from "masa," a corn based dough, and are widely consumed throughout the world. Their flat shape is produced by continuous sheeting machines, i.e. "sheeters," commonly used in the food processing industry. Typically, sheeters utilize a pair of generally opposed cylindrical rollers, known as "sheeter rollers," separated by a small gap through which masa dough is roll-compressed and formed into flat, continuous masa sheets.
Due to the growing popularity of tortillas and tortilla chips, various developments in sheeter and sheeting system design have been made to improve production as well as quality of tortilla products. One such development has been to incorporate the use of pre-sheeting devices which prepare the masa for final sheeting by the sheeter rollers. Pre-sheeting devices operate to keep irregular clumps of masa from reaching the sheeter rollers by initially forming them into first drafted masa sheets with a relatively uniform first thickness. These are then fed into the sheeter rollers to produce a final thickness, which is generally retained throughout subsequent processing steps, e.g. cutting, baking, packaging, etc.
An example of a pre-sheeting device is shown in U.S. Pat. No. 5,635,235 disclosing a method of handling masa. As can be seen in FIG. 7, the pre-sheeting device is a pair of cylindrical primary rollers 54 located upstream and arranged in series with a pair of cylindrical sheeter rollers 56 to reduce the thickness of masa dough in succession. The primary rollers 54 receive masa from a hopper 40 having counter rotating shafts 52 with rectangular projections 122 which drive the masa through a slot 116 and onto the primary rollers 54. Additionally, the '235 patent illustrates the use of a mixing extruder 16 which mixes and feeds a continuous stream of masa through a circular nozzle 22. The stream of masa is then broken into masa logs 74 for transfer and distribution to one or more hoppers.
Although not directed to masa pre-sheeters, another example of a pre-sheeting device is shown in U.S. Pat. No. 5,096,729 disclosing a blanching system for processing pasta. The blanching system has an extruder 12 which extrudes a pasta strand 13 from a kneader/sheeter 10 prior to being guided into a blanching tank 16. Although details of the extruder 12 is not disclosed, FIGS. 1, 2, and 4 show the extruder 12 as a pair of cylindrical sheet rollers similar to sheet rollers 14.
One drawback of the cylindrical roller-type pre-sheeting devices used in the '235 and '729 patents is the high costs associated with them. Typically, sheeter rollers are made from precision-machined steel cylinders having smooth, high-precision surfaces. The high-precision surfaces are necessary to achieve the typical sheet production thicknesses of less than 0.1 inch. Additionally, while these steel. cylinders are usually hollowed out to reduce weight, they remain sufficiently massive and heavy in order to exert the required pressure for sheeting. Because of their size and bulk, these steel cylinders typically require a large amount of space, as well as require larger, more powerful motors to rotate the steel rollers.
In addition to being costly, another drawback of cylindrical roller-type pre-sheeting devices is that they do not effectively condition and prime the masa to improve its cohesive strength. Because the granularity of masa is relatively coarse compared to flour dough, masa has little cohesive strength, and is thus easily breakable when formed into relatively thin sheets. Roller-type pre-sheeters typically do not improve masas ability to better withstand the stresses involved in later processing steps because the degree and duration of pressure applied to the masa is limited to the single pressure/pinch point in the gap between the sheeter rollers as the masa passes through the gap. This does not give the masa sufficient time to mix or organize its particle structure to thereby better maintain its shape and structure. Moreover, the single pinch point is typically insufficient to dissipate air bubbles trapped in the masa, which can adversely affect the consistency. Because of this inadequacy, additional mixing equipment is typically required, such as the mixing extruder 16 in the '235 patent, to pre-mix masa prior to being roll-sheeted. These additional equipment can increase the production costs substantially.
Another drawback of cylindrical roller-type pre-sheeter devices is that because masa dough is relatively sticky compared to flour dough, it is more likely to stick to the pre-sheeting roller surfaces, notwithstanding the use of blades 132 as shown in the '235 patent. Moreover, masa dough may accumulate on the blades 132 themselves.
Thus, while it is desirable to provide and incorporate a pre-sheeting device to a masa sheeting system, the present systems and methods of pre-sheeting may be inadequate to provide efficient, durable, and consistent production of quality flattened masa food products. There is a need to provide a pre-sheeting device or system capable of consistently producing first drafted masa sheets having a uniform thickness, to ensure a quality final product without straining or jeopardizing the production equipment.